U.S. patent application number 13/668193 was filed with the patent office on 2013-08-15 for optical dispersion compensation module using fiber bragg grating with multiple degrees of freedom for the optical field.
This patent application is currently assigned to NEC Laboratories America, Inc.. The applicant listed for this patent is NEC Laboratories America, Inc.. Invention is credited to Eduardo Mateo, Ting Wang, Lei Xu, Fatih Yaman, Shaoliang Zhang.
Application Number | 20130209035 13/668193 |
Document ID | / |
Family ID | 48945599 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130209035 |
Kind Code |
A1 |
Yaman; Fatih ; et
al. |
August 15, 2013 |
OPTICAL DISPERSION COMPENSATION MODULE USING FIBER BRAGG GRATING
WITH MULTIPLE DEGREES OF FREEDOM FOR THE OPTICAL FIELD
Abstract
Systems and methods are disclosed for enhancing optical
communication by performing dispersion compensation in an optical
fiber using a fiber Bragg grating (FBG); and providing increased
degrees of freedoms (DOFs) to distinguish forward and backward
propagating fields with a passive component.
Inventors: |
Yaman; Fatih; (Monmouth
Junction, NJ) ; Mateo; Eduardo; (Tokyo, JP) ;
Xu; Lei; (Princeton Junction, NJ) ; Zhang;
Shaoliang; (Plainsboro, NJ) ; Wang; Ting;
(West Windsor, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NEC Laboratories America, Inc.; |
|
|
US |
|
|
Assignee: |
NEC Laboratories America,
Inc.
Princeton
NJ
|
Family ID: |
48945599 |
Appl. No.: |
13/668193 |
Filed: |
November 2, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61554737 |
Nov 2, 2011 |
|
|
|
Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G02B 6/29394 20130101;
G02B 6/34 20130101; G02B 6/29319 20130101 |
Class at
Publication: |
385/37 |
International
Class: |
G02B 6/34 20060101
G02B006/34 |
Claims
1. A method for optical communication, comprising: performing
dispersion compensation in an optical fiber using a fiber Bragg
grating (FBG); and providing increased degrees of freedoms (DOFs)
to distinguish forward and backward propagating fields with a
passive component.
2. The method of claim 1, wherein the DOFs include a number of
modes.
3. The method of claim 1, comprising providing multiple cores of a
fiber with forward and backward travelling fields are placed on
different cores of the fiber and they can be distinguished based on
the core that they occupy
4. The method of claim 1, comprising providing different fibers
joined by directional couplers, wherein forward and backward
travelling fields are placed on different predetermined fibers and
distinguished based on the predetermined fiber.
5. The method of claim 1, wherein zero circulator is used with the
FBG.
6. The method of claim 1, wherein the FBG is written on multi-mode
fibers.
7. The method of claim 1, wherein the FBG is written on few-mode
fibers (FMFs).
8. The method of claim 1, wherein the FBG converts a forward
travelling optical field traveling in the same direction from one
mode to another mode.
9. The method of claim 1, wherein the FBG converts a forward
travelling optical field traveling in an opposite direction to a
different mode.
10. The method of claim 1, wherein the FBG converts a forward
travelling optical field traveling in an opposite direction to
leave the optical field in the same spatial mode but reflected in
an opposite direction.
11. A communication system, comprising: one or more optical fibers;
and means for writing a fiber Bragg grating (FBG) with dispersion
compensation through increased degrees of freedoms (DOFs) to
distinguish forward and backward propagating fields.
12. The system of claim 1, wherein the DOFs include a number of
spatial modes.
13. The system of claim 1, wherein the DOFs include multiple cores
of a fiber with forward and backward travelling fields are placed
on different cores of the fiber and they can be distinguished based
on the core that they occupy
14. The system of claim 1, wherein the DOFs use different fibers
joined by directional couplers, wherein forward and backward
travelling fields are placed on different predetermined fibers and
distinguished based on the predetermined fiber.
15. The system of claim 1, wherein the FBG is written without a
circulator.
16. The system of claim 1, wherein the FBG is written on multi-mode
fibers.
17. The system of claim 1, wherein the FBG is written on few-mode
fibers (FMFs).
18. The system of claim 1, wherein the FBG writing converts a
forward travelling optical field traveling in the same direction
from one mode to another mode.
19. The system of claim 1, wherein the FBG writing converts a
forward travelling optical field traveling in an opposite direction
to a different mode.
20. The system of claim 1, wherein the FBG writing converts a
forward travelling optical field traveling in an opposite direction
to leave the optical field in the same spatial mode but reflected
in an opposite direction.
Description
[0001] This application claims priority to U.S. Provisional
Application Ser. No. ______ filed on Nov. 2, 2011, the content of
which is incorporated by reference.
BACKGROUND
[0002] The present invention relates to Optical Dispersion
Compensation.
[0003] One of the main sources of signal distortion in optical
fibers is chromatic dispersion. Since dispersion has a linear
process and affects the signal phase, it can be perfectly
compensated without inducing any penalties. There are three main
approaches to overcoming distortions due to fiber dispersion.
First, reducing link dispersion, and second, compensating for
dispersion effects at the coherent receiver or at the digital
transmitter using digital signal processing.
[0004] The first approach can be implemented by designing fibers
that have almost no dispersion, such as dispersion shifted fibers.
However, it is well known if fiber dispersion is reduced, nonlinear
impairments increase significantly. Since it is much more easy, and
less expensive to compensate for fiber dispersion, than
compensating nonlinear impairments, this method has been largely
discarded by the community.
[0005] Another way to reduce link dispersion is concatenating
different fibers in the link such that the dispersion of one fiber
compensates totally or partially the dispersion of the other fiber.
Such links are called compensated links. However, the fibers having
negative dispersion has higher losses and higher nonlinearities.
Moreover, since the dispersion of the link is periodically
compensated, similar to the case of using low dispersion fibers,
these links also have higher nonlinear penalties compared to using
uncompensated links.
[0006] The second approach is to use uncompensated link with
coherent transceivers, and compensate the accumulated dispersion at
the transmitter side or at the receiver side using digital signal
processing (DSP). The second approach has two problems. First, link
dispersion can be very large, and current DSP technologies may not
be adequate for compensating for the accumulated dispersion of
transoceanic links. Second, compensation of dispersion by DSP
requires very large amount of power consumption. Indeed, even in
current receivers, majority of the power consumption is due to
dispersion compensation.
[0007] The third approach attempts to compensate for the total
dispersion of the entire link at the receiver side or at the
transmitter side by optical methods. Basically, a passive optical
component having the opposite of the accumulated dispersion of the
entire link can be placed at the transmitter or at the receiver to
fully compensate for the entire link dispersion. This is sometimes
referred to as lumped dispersion compensation (400). In this case
signal does not experience increased nonlinear penalty during
transmission because fiber has large dispersion. However, so far
there is no optical component that can compensate for the
accumulated dispersion of an uncompensated link more than a few
hundred kilometers meters long with acceptable insertion loss.
[0008] Since dispersion is a linear effect, in principle these
components can be cascaded many times to achieve the amount of
total dispersion that is required. However, because of the large
insertion loss and also large PDL of these components, cascading
them require using additional optical amplifiers which both
increases system cost and reduces signal quality because of
additional amplifier noise, and additional polarization-dependent
loss.
[0009] FIG. 1 shows one approach where optical components use fiber
Bragg grating (FBG) for dispersion compensation (100). Bragg
gratings (BGs) can be written on many media, and when they are
written on fibers they are called FBGs. FIG. 1 shows a simple
schematic of how FBGs are typically used for dispersion
compensation. The dispersed light enters from the left port of the
circulator (101) and it is directed to the FBG (102). The short
wavelengths have a higher group velocity, therefore they are ahead
of the other wavelengths, and longer wavelengths are trailing. The
BG is written with a chirp in the pitch, meaning that the pitch
changes along the grating so that a phase matching condition is
satisfied for longer wavelengths at the near side of the BG but not
for the shorter wavelengths. Hence, the longer wavelengths are
reflected from the nearside of the FBG. Similarly, the pitch is
designed so that it satisfies the phase matching condition for
shorter wavelengths at the far side of the BG. As a result, if the
pitch and the chirp in the pitch is controlled well enough, the
shorter wavelengths can be delayed with respect to the longer
wavelengths by just the right amount so that the dispersion can be
compensated.
[0010] There are two limitations to this method. First the largest
dispersion amount that can be compensated depends on how long the
FBG can be written, and the maximum length is currently limited to
only a few meters due to the limitations of the FBG writing
techniques. Second, FBG is reflective. Therefore it is necessary to
use a circulator, which increases the insertion loss by at least
1.7 dB, because of the dual pass through the circulator. Indeed,
the circulator is the major contributor to the insertion loss since
the loss of a few meter long fiber can be ignored and writing FBG
does not increase fiber loss appreciably. Assuming a maximum of a 1
m long FBG, the maximum amount of dispersion that can be
compensated for is only 10000 ps, which is far less than 50000 to
150000 ps which is required for typical uncompensated transoceanic
links. Cascading several FBGs can achieve the desired total amount
of dispersion compensation at the cost of increased insertion
loss.
SUMMARY
[0011] Systems and methods are disclosed for enhancing optical
communication by performing dispersion compensation in an optical
fiber using a fiber Bragg grating (FBG); and providing increased
degrees of freedoms (DOFs) to distinguish forward and backward
propagating fields with a passive component.
[0012] Implementations of the above aspect can include one or more
of the following. The DOFs include a number of spatial modes. The
system can provide multiple cores of a fiber with forward and
backward travelling fields are placed on different cores of the
fiber and they can be distinguished based on the core that they
occupy. Different fibers can be joined by directional couplers,
wherein forward and backward travelling fields are placed on
different predetermined fibers and distinguished based on the
predetermined fiber. No circulator is used with the FBG. The FBG
can be written on multi-mode fibers or on on few-mode fibers
(FMFs). The FBG converts a forward travelling optical field
traveling in the same direction from one mode to another mode. The
FBG converts a forward travelling optical field traveling in an
opposite direction to a different mode. The FBG converts a forward
travelling optical field traveling in an opposite direction to
leave the optical field in the same spatial mode but reflected in
an opposite direction.
[0013] Advantages of the above aspect may include one or more of
the following. The system attempts to compensate for the total link
dispersion without increasing link loss, nonlinear penalties and
without using digital signal processing at the transceiver which
increases power consumption of the transceivers significantly. The
system also attempts to achieve dispersion compensation using low
cost materials with low insertion loss, low polarization-dependent
loss (PDL), compact packaging, improved insensitivity to
environment and higher power handling capability. The system
achieves high performance due to reduced insertion loss and no PDL.
The system can compensate larger amount of dispersion for a given
length of fiber. The system has less complexity because it is an
all fiber device that does not require the use of circulators. The
system uses less components, and therefore easier for packaging,
and for assembly. Lower cost because it has less components, and
easier to package, and it has a small form factor. The system
achieves higher operational range, since it does not use
circulator, and it is a fiber only device, it can handle much
larger optical power. The insertion loss of FBG based dispersion
compensator is reduced significantly, as the present invention does
not require circulator which is the main source of insertion loss.
The PDL of the FBG based dispersion compensator is reduced
significantly as the present invention does not require circulator
which is the main source of polarization-dependent loss. The
maximum amount of dispersion compensation achievable by a given
length of fiber is reduced significantly by making use of multiple
spatial modes, and the possibility of superimposing multiple BGs on
the same stretch of fiber. The maximum amount of dispersion
compensation achievable by a given length of fiber is reduced
significantly by making use of multiple spatial modes, but using
less number of BGs than the number of spatial modes used, by
engineering the modal propagation constants of the fiber. The
maximum amount of dispersion compensation achievable by a given
length of fiber is reduced significantly by making use of multiple
spatial modes, but using less number of BGs than the number of
spatial modes used, by making use of the spatial symmetry of the
modes. The packaging size is reduced because the device does not
require the use of circulator and because of the shorter length of
fiber required to compensate a given amount of dispersion. The
device is less complex because it does not require any additional
component than fiber. The device has increased tolerance for
optical power since it does not require the use of circulator which
limits the maximum amount of power that can be handled by the
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a simple schematic of how FBGs are typically
used for dispersion compensation.
[0015] FIG. 2 shows a first embodiment of an Optical Dispersion
Compensation Module Using Fiber Bragg Grating with Additional
Multiple Degrees of Freedom for the Optical Field.
[0016] FIG. 3 shows a multi-mode embodiment.
[0017] FIG. 4 shows a lumped optical dispersion compensation
system.
DETAILED DESCRIPTION
[0018] FIG. 2 shows a device with an Optical Dispersion
Compensation Module Using Fiber Bragg Grating With Additional
Multiple Degrees of Freedom For the Optical Field. The device
consists of a FMF (200), the rectangle surrounding the figure, with
three consecutive Bragg gratings (BGs) (201, 202, 203). Optical
field that will experience dispersion compensation is launched to
the FMF from the left side, and it leaces the fiber from the right
end after proper dispersion compensation.
[0019] Initially the field is launched from the left side and it is
only coupled to the mode 1, for instance to the fundamental mode.
The field propagates unchanged until the BG2 (202). This is
guaranteed by making sure that BG1(201) is written with grating
period such that it does not phase match any frequency component of
mode 1 to any mode supported by the fiber, or the cladding, or the
radiation modes. In other words, if k.sub.1 is the wavenumber of
the BG1, k.sub.1.noteq..beta..sub.1-.beta..sub.n for any possible
.beta..sub.n, where .beta..sub.1 is the propagation constant of
mode 1.
[0020] After passing through the BG1 unchanged, field arrives at
BG2 which is designed so that it satisfies the phase matching
condition between forward propagating mode 1 to backward
propagating mode 2. Also, BG2 is chirped so that, different
frequency components of the field is reflected and mode converted
at different locations along BG2 depending on the desired
dispersion. As a result, after BG2 mode 1 is reflected back,
converted into mode 2 and experiences dispersion.
[0021] After BG2, field travels back to BG1 where it is reflected
forward, and converted to mode 3 and also experiences dispersion by
the desired amount, at the same time. Mode 3 passes through BG2
unchanged and without loss because BG2 does not satisfy
phase-matching between any confined or unconfined fiber mode and
mode 3.
[0022] After BG2, if desired, an additional mode convertor can be
used to convert mode 3 back to mode 1. This is typically the case
where the field entering the device and leaving the device would
have the same spatial mode, but it is not necessary for the
dispersion compensation. If this is the case, an additional BG
(203) can be used to convert mode 3 back to mode 1 but leaves it
propagating in the forward direction.
[0023] The difference between the propagation constants of forward
propagating mode 1 and backward propagating mode 2 should be
different than the difference between the propagation constants of
any possible mode and mode 1, 2 or 3. Similarly, the difference in
the propagation constants of backward propagating mode 2 and
forward propagating mode 3 should be different than the difference
between the propagation constants of any possible mode and mode 1,
2, or 3. In other words,
k.sub.2=(|.beta..sub.1|+|.beta..sub.3|).noteq.|.beta..sub.j.+-..beta..sub-
.m|, for any pair of j and m that includes j=1,2,3 and any m other
than obviously the pair j=1, and m=3. Fiber should also be designed
so that,
k.sub.3=(|.beta..sub.2|).noteq.|.beta..sub.j.+-..beta..sub.m| for
any pair of j and m that includes j=1,2,3 and any m other than pair
j=2, and m=3. Here .+-..beta..sub.m corresponds to the propagation
constant of the m.sup.th mode travelling in the forward or backward
direction, respectively, and k.sub.mcorresponds to the wave-vector
of the m.sup.th BG. The FBGs are designed to operate over a given
frequency range. It is assumed that the above conditions are
satisfied for all the frequencies in the given range. These
conditions can be easily satisfied for many types of standard
multimode fibers (408) but especially for FMFs (407), because the
differences in the propagation constants of different modes in FMFs
are significantly larger compared to multimode fibers. Therefore it
is easier to design these fibers so that it is possible to write
many different BGs with different phase matching conditions that do
not overlap for a large frequency range. FIG. 2 gives the example
of using only 3 modes, but this can be generalized so that many
more modes can be utilized to add more and more dispersion
compensation. An example of how this device can be generalized is
given in FIG. 3 which shows a multi-mode embodiment.
[0024] In FIG. 3, boxes (303) represent BGs that convert one mode
to the other but does not change the propagation direction. Boxes
(301) and (302) represent BGs that simultaneously convert one mode
to the other, invert the direction of propagation, i.e., reflect,
and also impart dispersion. The only difference between the boxes
301 and 302 is the location, i.e., one is at the near side of the
fiber and the other is at the back side. Even though the boxes are
drawn in separate locations, and modes are separated for clarity,
the BGs 301 can be super-imposed, and the BGs 302 can also be super
imposed. The BGs shown in the red and blue boxes cannot overlap.
The BG shown by 303 can overlap with the BGs 302 but not with the
BGs 303. Being able to super impose these BGs (410) has the
advantage that using a short piece of fiber, the amount of
dispersion that can be compensated grows with the number of spatial
modes (406) that are used.
[0025] In one embodiment, BGs 301 and the BGs 302 satisfy
additional phase matching conditions. None of the BGs 301 should
satisfy a phase matching between any of the modes travelling in the
backward direction and any possible mode of the fiber except for
the particular modes that it is coupling. In other words, for the
grating with the grating wave-number k.sub.23, i.e. blue grating
301 that converts mode 2 to mode 3, the fiber should be designed so
that
k.sub.23=(|.beta..sub.2|+|+|.beta..sub.3|).noteq.|.beta..sub.j.+-..beta..-
sub.m|, for any pair of j and m that includes j=2,4,6 . . . , N-1
and any m other than the intended pair of j=2 , and m=3. The
conditions for other blue gratings 301 can be found by replacing, 2
and 3 by the appropriate pairs such as 4 and 5, 6 and 7 etc.
Similarly, none of the BGs 302 on the right should satisfy
phase-matching condition between any of the forward travelling mode
and any possible mode except the particular modes that it is
intended to convert. In other words, for the grating with the
grating wave-number k.sub.12 that converts mode 1 to 2, the fiber
should be designed so that
k.sub.12=(|.beta..sub.1|+|.beta..sub.2|).noteq.|.beta..sub.j.+-..beta..su-
b.m|, for any pair of j and m that includes j=1,3,5, . . . ,N and
any m other than the intended pair of j=1, and m=2. The conditions
for other red gratings 302 on the right can be found by replacing,
1 and 2 by the appropriate pairs such as 3 and 4, 5 and 6 etc.
Finally the BG that converts mode N back to mode 1 on the right
most part of the fiber (box 303) should satisfy the phase matching
condition
k.sub.N1=.parallel..beta..sub.N|-|.beta..sub.1.parallel..
[0026] The above conditions implies that if one uses N modes, and
use 1 BG per mode, one can achieve (N-1)/2 times more dispersion
compensation using the same length of fiber, because, the
conversion from mode N back to mode 1 does not contribute to
dispersion compensation, and the BGs 301 and 302 cannot overlap
requiring them to utilize half the fiber length (410).
[0027] Ideally, an indefinite number of BGs can be super-imposed
(410), since the impulse response of the BGs is linear. However, as
N BGs are superimposed at the same location, the maximum
peak-to-valley deviations in the refractive index perturbation also
increases linearly with N. In practice, the maximum amount of
perturbation achievable in refractive index is limited. Therefore,
the number of BGs that can be superimposed is limited, and
therefore the number of additional modes that can be utilized is
limited. Therefore, the maximum amount of dispersion that can be
compensated by a given length of FBG is limited.
[0028] One embodiment avoids this limitation by relaxing the above
conditions for the propagation constants of different modes. The
conditions given above are sufficient but not necessary depending
on the fiber design. Note that, the same red BG can be used to
convert mode 1 to mode 2, and mode 3 to mode 4, at the same time if
the fiber can be designed so that the difference in the propagation
constants of these pairs are equal (411). In other words,
k.sub.12=(|.beta..sub.1|+|.beta..sub.2|)=(|.beta..sub.3|+|.beta..sub.4|).
In this case, one BG can be avoided without sacrificing from the
total amount of dispersion compensation. In the extreme case, it is
possible to use as many modes as possible by using a total of only
one red BG one blue BG and one mode convertor at the right side as
long as the fiber modes satisfy the following conditions (411);
k.sub.12=(|.beta..sub.1|+|.beta..sub.2|)=(|.beta..sub.3|+|.beta..sub.4|)=
. . . =(|.beta..sub.N-2|+|.beta..sub.N-1|) and for the blue BGs
k.sub.23=(|.beta..sub.2|+|.beta..sub.3|)=(|.beta..sub.4|+|.beta..sub.5|)=
. . . =(|.beta..sub.N-1|+|.beta..sub.N|), but
(|.beta..sub.1|+|.beta..sub.2|).noteq.(|.beta..sub.2|+|.beta..sub.3|),
and also
.parallel..beta..sub.N|-|.beta..sub.1.parallel..noteq.(|.beta..s-
ub.2|+|.beta..sub.3|).
[0029] Another embodiment relaxes the conditions that the fiber
modes need to satisfy (412) is that making use of the fact that if
both of the pair of coupled modes are symmetric, or both
asymmetric, a grating with a refractive index profile perpendicular
to the fiber core is the most efficient way to couple them. If one
of them is symmetric and the other is asymmetric, a grating with a
tilted index profile is the most efficient way to couple them. Such
gratings are also known as blazed gratings (412). For instance,
instead of requiring two mode pairs to satisfy the condition
(|.beta..sub.1+|.beta..sub.2|).noteq.(|.beta..sub.2+.beta..sub.3|),
it is possible to let them
(|.beta..sub.1|+|.beta..sub.2|)=(|.beta..sub.3|+|.beta..sub.4|) as
long as one and only one of the 4 modes in this equation has a
different spatial symmetry than the other three. In this case, the
different mode pairs can be manipulated separately based on their
spatial symmetries and not on their propagation constant
differences.
[0030] Even though the above descriptions are based on using
different spatial modes of multimode (408) and few-mode fibers
(407) as additional degrees of freedom, it is clear that other
degrees of freedoms can be utilized instead of spatial modes or
together with spatial modes. For instance, it is possible to
reflect the field into a different fiber core together with
dispersion compensation if the fiber has multiple cores (404). It
is also possible to reflect it back to a separate fiber by using
fiber couplers along with gratings or other passive nonreciprocal
components (405). However, using spatial modes is the most
practical method as all the fields propagate in the same fiber core
which makes it easier to write gratings, and also fields retain
their coherence better (406).
[0031] FBG are obtained by generating perturbations in the
refractive index profile of the fiber. These perturbations can
cause mode coupling between different modes of the fiber. Here the
fiber modes are used in the most general definition which includes
all the modes confined to the core, that are confined in the
cladding, those that are leaky, that are not confined by the fiber
such as radiation modes, and also modes that travel in the forward
or backward direction. In the simplest case, these perturbations
are written on along the fiber with a very well controlled
periodicity, which can couple a forward propagating mode, to a
backward propagating mode, which is just reflection of the mode.
FBGs can also couple one spatial mode of the fiber to the other.
These modes can travel in the same direction or in the reverse
direction [2]. The mode coupling depends on two conditions, one the
two modes have non-zero overlap, and second they need to satisfy
the phase-matching condition, given by
k(z)=.beta..sub.1(.omega.)-.beta..sub.2(.omega.) (1)
where k(z)=2.pi./.LAMBDA.(z) is the local grating wave number,
.LAMBDA. is the local pitch of the perturbation, .beta..sub.1 and
.beta..sub.2 are the propagation constants of the coupled modes,
which can take negative value for backward propagating modes.
[0032] In current FBGs used for dispersion compensation,
single-mode fiber (SMF) is used (402), which means, the fiber core
supports propagation of only one spatial mode, which can propagate
either in the forward or backward direction. The forward
propagating field and the backward propagating fields occupy the
same spatial mode, same polarization modes, and same stretch of
fiber. The only difference is that one is propagating in the
forward direction and the other in the backward direction. For this
component to work, it is necessary to include a component that will
allow the forward propagating field to pass through but reflect the
backward propagating field coming back from the grating. The only
passive components that can distinguish backward and forward
propagating fields that otherwise occupy the same spatial mode,
same polarization, and same space are called non-reciprocal
components. The non-reciprocal components of choice are the
circulators (102). However, using circulators significantly
increase the insertion loss, causes PDL, increases costs, and
complicates packaging.
[0033] In one embodiment, the system increases the degree of
freedoms (DOFs) (403) such as the number of modes (406) so that we
can place the forward and backward propagating field in different
DOFs such as different spatial modes, and use a simpler, linear,
passive, and reciprocal devices that can guide the forward and
backward propagating fields in the same or different directions
without increasing insertion los significantly, causing PDL, among
others. Another example of possible DOFs that can be used is
multiple cores of a fiber (404). In this case forward and backward
travelling fields can be placed on different cores of the fiber and
they can be distinguished based on the core that they occupy.
Another example of possible DOFs that can be used is using
different fibers joined by directional couplers (405). In this case
the forward and backward travelling fields can be placed on
different fibers and they can be distinguished based on the fiber
that they travel in.
[0034] The system solves the problem of high insertion loss, by
avoiding the use of circulator (403) which is the main source of
insertion loss, and PDL by using devices that provide additional
DOFs for the propagating field. In one of the possible
implementations of the current invention the FBG is written on
multi-mode fibers (406), in particular on few-mode fibers (FMFs)
(407). Another advantage of using FMFs (407) compared to multimode
fibers (408) is that they suffer less from unwanted modal couplings
compared to multimode fibers.
[0035] Current dispersion compensating FBGs are written on
single-mode fibers (402). These fibers support propagation of light
in only a single-spatial mode. Multimode fibers on the other hand
support propagation of optical field in multiple spatial modes.
FMFs are a subset of multimode fibers such that they support only a
few modes, typically less than 10, sometimes less than 50, as
opposed to multimode fibers which support very large number of
modes.
[0036] FBGs are passive devices; therefore they cannot
differentiate a forward propagating field from a backward
propagating field. If multimode fibers are used for FBGs, a
distinction can be made between forward and backward travelling
fields by making sure that they are in different spatial modes.
This gives an additional degree of freedom to design FBGs which can
make it possible to avoid using circulators.
[0037] One embodiment uses shorter lengths of fiber to achieve the
same amount of total dispersion compensation. It is possible to
write FBGs that will affect a particular spatial mode and not
others. Moreover, the FBGs that affect different spatial modes can
be superimposed on the same stretch of fiber (410). Once the field
is launched into the device, it can reflect back and forth, each
time travelling in a different spatial mode, and each time
accumulating dispersion from the FBG written for that particular
mode until it is passed to the final spatial mode which is not
reflected back and allowed to leave the device.
[0038] Another embodiment uses FBGs to convert a forward travelling
optical field from one mode to another mode also traveling in the
same direction, or to convert it to a different mode travelling in
the opposite direction, or leave it in the same spatial mode but
reflect in the opposite direction.
[0039] Long multimode fibers suffer from mode mixing. However, mode
mixing is very small for fiber lengths that would be used for FBGs.
It was recently shown that in FMFs (407) mode mixing is as low as
-18 dB after 40 km, which would make it as low as 64 dB for a
1-meter-long fiber [4], which is sufficiently low enough to be
neglected. The system of FIG. 4 achieves the following: [0040] (1)
Making use of additional degrees of freedom (DOF) to avoid using
circulators, and also increase the amount of dispersion
compensation per a given fiber length. [0041] (2) Such DOFs can be
as follows; different spatial modes of multimode fibers, different
cores of a multi-core fiber, or different fibers joined by
directional coupler. [0042] (3) Using FMFs as opposed to the
standard multimode fibers because the propagation constants of the
modes of FMF have much better separation which makes it possible to
use more modes for compensation. Also FMFs suffer less from
unwanted mode mixing compared to standard multimode fibers. [0043]
(4) Using single BG to do mode conversion and dispersion
compensation and reflection of different mode pairs to reduce the
number of required BGs to take advantage of multiple modes, by
using fibers with appropriate mode properties. [0044] (5) Making
use of the spatial symmetries of modes to relax the conditions that
different mode pairs need to satisfy, so that more modes can be
used.
[0045] The system of FIG. 4 has much better performance,
because:
[0046] 1. It has reduced insertion loss.
[0047] 2. It does not have PDL.
[0048] 3. It can compensate larger amount of dispersion for a given
length of fiber.
[0049] 4. It has less complexity because it is an all fiber device
that does not require the use of circulators.
[0050] 5. It has less components, and therefore easier for
packaging, and for assembly.
[0051] 6. Lower cost because it has less components, and easier to
package, and it has a small form factor.
[0052] 7. Higher operational range, since it does not use
circulator, and it is a fiber only device, it can handle much
larger optical power.
[0053] The system attempts to compensate for the total link
dispersion without increasing link loss, nonlinear penalties and
without using digital signal processing at the transceiver which
increases power consumption of the transceivers significantly. The
system also attempts to achieve dispersion compensation using low
cost materials with low insertion loss, low polarization-dependent
loss (PDL), compact packaging, improved insensitivity to
environment and higher power handling capability. The system
achieves high performance due to reduced insertion loss and no PDL.
The system can compensate larger amount of dispersion for a given
length of fiber. The system has less complexity because it is an
all fiber device that does not require the use of circulators. The
system uses less components, and therefore easier for packaging,
and for assembly. Lower cost because it has less components, and
easier to package, and it has a small form factor. The system
achieves higher operational range, since it does not use
circulator, and it is a fiber only device, it can handle much
larger optical power. The insertion loss of FBG based dispersion
compensator is reduced significantly, as the present invention does
not require circulator which is the main source of insertion loss.
The PDL of the FBG based dispersion compensator is reduced
significantly as the present invention does not require circulator
which is the main source of polarization-dependent loss. The
maximum amount of dispersion compensation achievable by a given
length of fiber is reduced significantly by making use of multiple
spatial modes, and the possibility of superimposing multiple BGs on
the same stretch of fiber. The maximum amount of dispersion
compensation achievable by a given length of fiber is reduced
significantly by making use of multiple spatial modes, but using
less number of BGs than the number of spatial modes used, by
engineering the modal propagation constants of the fiber. The
maximum amount of dispersion compensation achievable by a given
length of fiber is reduced significantly by making use of multiple
spatial modes, but using less number of BGs than the number of
spatial modes used, by making use of the spatial symmetry of the
modes. The packaging size is reduced because the device does not
require the use of circulator and because of the shorter length of
fiber required to compensate a given amount of dispersion. The
device is less complex because it does not require any additional
component than fiber. The device has increased tolerance for
optical power since it does not require the use of circulator which
limits the maximum amount of power that can be handled by the
device.
[0054] An optical network incorporating the optical dispersion
compensation apparatus and methods described can more readily
accommodate time-varying traffic patterns, data rates, or other
network parameters as well as different standard data rates and
foreign optical wavelengths. Bandwidth is more efficiently used
throughout the network and the need for transponder regeneration is
reduced. Utilization of the described methods and apparatus can be
used to eliminate optical dispersion, such as chromatic dispersion,
as a network design restriction or other engineering parameter.
Accordingly, overall network design, deployment, maintenance, and
operation are simplified.
[0055] Methods and apparatus for providing dispersion compensation
have been described. Various modifications and changes may be made
thereto without departing from the broader scope of the invention
as set forth in the claims. The specification and drawings are,
accordingly, to be regarded in an illustrative rather than a
restrictive sense.
* * * * *